For BASE firmware updating you need to copy firmware file (xgf) to the X6100 file system. There are few way to do it. One of them is described
here
Make sure that X6100 is booted with OEM firmware (without inserted SD card) and connected to WiFi before copying base firmware file.
After copying new files should appear in the system -> firmware upgrade menu. Last step - select desired firmware and press upgrade.
?
Perhaps, I'll inspect BASE flashing process and will be able to implement it within R1CBU in the future.
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Georgy // R2RFE